The usual time difference method in ultrasonic flowmeters involves the transmission of ultrasonic waves through a fluid medium in two directions, one upstream and the other downstream of the direction of flow, and comparing the transit times, normally over paths of equal lengths. Assuming that the speed of sound remains constant, the speed of propagation of the waves in the fluid medium is the same over both paths; and the transit time varies according to the velocity of the fluid medium which shortens the transit time over the downstream path and lengthens the transit time over the upstream path. From the difference between the upstream and downstream transit times, the flow velocity of the liquid medium can be calculated by a time difference technique.
The basic theory of the time difference technique can be explained as follows: Assume, for example, that a conduit having a uniform flow of fluid of velocity, v, contains two sets of transducers facing each other at a spacing, d, between the two. If the sound velocity in the fluid at rest is c, then the respective transit times downstream, t.sub.A, and upstream, t.sub.B, can be represented as follows: ##EQU1## AND ##EQU2## If a pulse is simultaneousy transmitted in both paths, the received signals arrive at times differing by: ##EQU3## The velocity of the fluid, v, is almost always much smaller than the sound velocity, c, in the fluid at rest for all practical applications in liquids. For these applications, the approximate equation: ##EQU4## IS SUFFICIENTLY ACCURATE. Thus, .DELTA.t is proportional to v with the calibration constant being a function of c, and v can be determined from the relationship: EQU v = (1/2d)c.sup.2 .DELTA.t (5)
Where 1/2d is a constant.
A problem arises, however, in that the speed of sound, in reality, is not constant but varies with the mineral content and temperature of the fluid being measured. This is particularly true when measuring the flow of a fluid in an open unrestricted channel of flow for instance, at sea.
Means have been proposed in order to compensate for the varying speed of sound. One type of compensating scheme is referred to in U.S. Pat. No. 3,440,876 entitled "Flowmeter Frequency Control System" of W. E. Hayes and C. D. Calhoun. The scheme utilizes a variable frequency oscillator (VFO) and a counter device which is responsive to the VFO output for providing a marker output signal when a certain count has been reached. The provision of this marker pulse is obtained by initially choosing a nominally correct speed of sound, determining how long it should take acoustic energy to travel the distance between transducer stations in the absence of fluid velocity, and knowing the frequency of the VFO, the marker pulse should occur at the same time the acoustic energy is received, and the count in the counter provides an indication of this situation. The VFO also feeds a time interval counter which is turned on, when the downstream acoustic energy is received, to provide a first received pulse, or signal, and is turned off, when the upstream acoustic energy is received, to provide a second received pulse, or signal. The count in the time interval counter is indicative of the fluid velocity. In order to control the frequency of the VFO, the marker pulse is compared with the first received signal.
As an improvement, Hayes uses the VFO to provide pulses to a counter for a period of time .DELTA. T equal to the difference in transit time of the acoustic pulses travelling between upstream and downstream transducers. A first counter counts up the VFO output and provides a marker signal indicative of a received pulse in the absence of any fluid velocity. Another counter means stores a count indicative of one half the count in the .DELTA. T counter and is provided with the VFO output such that when full it will provide a comparison signal when the .DELTA. T is half full. The marker and comparison signals are utilized to control the frequency of the VFO.
In U.S. Pat. No. 3,653,259, of McShane issued Apr. 4, 1972, an ultrasonic flowmeter system based upon a time difference technique is described wherein the time delay between acoustic pulses transmitted upstream and downstream in a fluid passing along a path of travel is multiplied by repeated transmissions in sing-around fashion. This has the advantage of obviating the necessity for measuring very small differences between upstream and downstream transmissions to determine velocity, by merely using the cumulative effect in combining the time delays between upstream and downstream pulses over a chosen period of time.
Thus, the output from the system shown in U.S. Pat. No. 3,653,259 is proportional to the total time included in a series of ever-widening pulses. This technique will be referred to hereinafter as the McShane multiple time difference technique, in order to distinguish over those techniques in which only one time difference .DELTA.t is measured. As explained in the McShane patent, instead of measuring a time difference .DELTA.t between a single pair of received pulses, which would be very small, the quantity .DELTA.t is expanded by repeated transmissions in sing-around fashion. Regenerative sing-around action is effected during a selected time interval T, the first transmitted pulse being generated in each loop. The sing-around loops continue to run for the chosen time T while counting for N pulses during such time interval T in each loop. As a result an output signal representing N.DELTA.t is derived instead of only .DELTA.t in amplitude.
However, since the McShane method only amplifies what each .DELTA.t provides, the same dependency upon the speed of sound c in the fluid exists in the N.DELTA.t derived signal. McShane therefore provides for "c-correction" of the N.DELTA.t pulse signal. Thus the McShane patent describes an improvement consisting in using the sum of the ever widening signals .DELTA.t rather than the last signal N.DELTA.t. This technique will be hereinafter called the "summation multiple time difference" (.SIGMA.N.DELTA.t) as opposed to the original multiple time difference (N.DELTA.t) technique of McShane. Therefore, when performing the summation of all of the pulses in the multiple pulse train, all N.DELTA.t pulses are in fact successively measured. Also, this sound velocity correction method is affected by an uncertainty due to a pulse edge effect appearing at the end of the train.
Another prior art mode of correction is described in U.S. Pat. No. 3,546,935 entitled "Fluid Measurement System and Method" of P. J. Bruha. This patent also utilizes a variable frequency oscillator to produce a frequency which is dependent upon the speed of sound, and uses this (VFO) frequency to correct the .DELTA.t in the conventional technique, or the N(.DELTA.t) in the multiple time difference technique of McShane. The difficulty with this mode of compensating for the speed of sound results from the fact that actually a separate measurement of the speed of sound is made. Such a measurement requires at least as much power as for a .DELTA.t measurement. In fact the VFO must run continuously. This is a waste of power.
It is therefore one object of the present invention to provide a system of the type described wherein correction for speed of sound variations is improved.
A further object is to provide a flowmeter system of the multiple time interval difference having an inherent compensation for variations in the speed of sound.
Another object of the present invention is to provide a sing-around flowmeter of improved accuracy and independent from the speed of sound.
The present invention being an improvement over the multiple sing-around flowmeter system described in U.S. Pat. No. 3,653,259 entitled "Ultrasonic Flowmeter Systems" of J. L. McShane, issued Apr. 4, 1972 and assigned to the same assignee as the assignee of this application, the text of the said patent is incorporated by reference in this application for the purpose of explaining more completely the supporting McShane multiple time difference technique as well as of complementing the description herein of the preferred embodiment of the present invention.